How to flip the function f(x) about y-axis

Suppose I have a function W(x). I wanted to flip (reflect) this function about the y-axis. I tried plotting W(-x) but it is not giving the expected results.

L = 4;
beta1 = {0.468776, 1.17352, 1.96369, 2.74889, 3.53429, 4.31969, 
  5.10509, 5.89049, 6.67588, 7.46128, 8.24668, 9.03208, 9.81748}
W = ((Cos(b*x) - 
      Cosh(b*x)) - (((Cos(b*L) + Cosh(b*L))/(Sin(b*L) + 
         Sinh(b*L)))*(Sin(b*x) - Sinh(b*x)))) /. b -> beta1((1))